Technology for manufacturing memory circuits (e.g., random access memory) involves complex semiconductor production processes. These processes are not perfect insofar as memory integrated circuits (ICs) are thoroughly tested after manufacture. One aspect of memory IC testing involves testing for operating conditions of wordlines and bitlines.
In older memory technologies, wordlines are driven from both sides of the local wordline. These are called stitched wordlines. To save wafer space in newer technologies, a segmented wordline concept was introduced that drives the wordlines only from one side. This single-sided driving configuration has the disadvantage that as soon as the driven side is partially disconnected the wordline is no longer driven, and therefore will float at an undefined voltage level.
Simulation has shown that there is a voltage range in which the floating wordline behavior is most problematic. For example, in one memory technology this range is 1.3V to 1.6V. This range may vary in other memory technologies and applications. In this volatile range, the floating wordline connected memory cells can destroy or contaminate data stored in good wordline connected memory cells.
Currently, there is no practical and accurate method to characterize and test for floating wordline conditions in a memory IC chip. The only way heretofore known to tune a floating wordline test for a particular production memory chip is to use a specially-designed failure application chip and adopt a known test with the failure application chip in order to detect a floating wordline condition. If the only faulty or missing connection in the chip is a connection between a pull-down driver circuit transistor to a negative wordline voltage, it is likely that only one floating wordline will be detected even though others may be present. As a consequence, a test can be tuned or adjusted only to a particular memory chip.
What is needed is a way to design tests for floating wordlines in a memory IC chip that overcomes the shortcomings of the existing techniques.